Add A2A v0.3 agent-card compatibility (legacy path + transport fields)

[MisterVVP]

Motivation

• TCK runs expect backwards-compatible Agent Card discovery for A2A v0.3.0, including legacy well-known path and transport-discovery fields, which were not being exposed.
• Provide helper fields so older discovery utilities can extract an endpoint, preferredTransport and additionalInterfaces from the modern supportedInterfaces representation.

Description

• Add kLegacyAgentCardPath and route /.well-known/agent.json to the same Agent Card handler as /.well-known/agent-card.json in RestServerTransport.
• Add FindInterfaceUrl helper and populate legacy-friendly endpoint and preferredTransport fields from supportedInterfaces when absent in the serialized JSON.
• Emit additionalInterfaces as a list of legacy transport entries (jsonrpc, rest, grpc) mapping each supported interface to an endpoint for v0.3 discovery helpers.
• Add unit tests verifying the legacy well-known endpoint responds and that the served Agent Card contains endpoint, preferredTransport, and additionalInterfaces fields.

Testing

• Ran code formatting via ./scripts/run_clang_format.sh which completed successfully.
• Built the project with cmake -S . -B build && cmake --build build which completed successfully and produced test binaries including rest_server_transport_test.
• Ran ./scripts/run_clang_tidy.sh build which reported repository-wide warnings/errors (not introduced by these changes) and failed in this environment.
• Ran ./scripts/verify_changes.sh and a targeted build/ctest invocation for rest_server_transport_test; the build succeeded and the new unit tests compiled and linked, and the Agent Card handler behavior validated locally via the added unit tests.

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Codex Task